Identification of a novel cytokine-like transcript differentially expressed in avian γδ T cells

Chicken represents a species with a high frequency of γδ T cells in peripheral blood, suggesting an important function. To elucidate the genes specific for the avian γδ T cells, the suppression subtractive hybridization (SSH) between γδ and αβ T cells was used. The SSH library, which was successfully enriched for the TCR γ and δ (both V and C region) sequences, provided γδ T-cell-specific genes, including, for example, the ribosomal proteins, signaling and structural molecules, and molecules related to transcription and translation. Among these genes, a clone named KK34 was shown to match the PFAM profile for IL-5 and to have 19.5% amino acid identity to the human interleukin 5 protein. Clone KK34 had lower sequence homology, from 5.4% to 15.6%, to other short-chain four-helix bundle superfamily members IL-3, IL-4, IL-13 and GM-CSF. The hydrophobic signal peptide sequence and the presence of cysteines needed for the interchain disulfide bonds were found to be conserved between clone KK34 and mammalian IL-5 proteins. Clone KK34 transcript expression was studied by RT-PCR, Northern blotting and in situ hybridization and it was confirmed to be expressed in avian γδ T cells. We propose that this clone, KK34, may represent the first nonmammalian IL-5, supporting the findings that γδ T cells are important in the development of allergy.